1. Field of the Invention
The present invention relates to the field of engine exhaust brake.
2. Description of the Background Art
Exhaust braking is an engine operating mode wherein the engine is reconfigured during operation to provide a braking effect to a vehicle. This may be desirable or necessary when regular wheel brakes are inadequate to provide complete braking. An example is a need for powerful and prolonged braking operations on steep grades, such as on mountain roads. Exhaust braking finds particular applicability on large vehicles having high wheel weights and correspondingly high momentum, and where conventional wheel brakes may fade or fail under high loading conditions or under prolonged use.
An engine brake works by opening exhaust valves at or near the end of the compression stroke of an associated cylinder. During the compression stroke of an engine, the air in a cylinder is compressed, requiring a work input by the engine. In normal engine operation the combustion stroke follows the compression stroke and recoups the work expended during the compression stroke. The opening of the exhaust valve near the end of the compression stroke means that no expansion of the compressed air occurs, with the air being exhausted from the engine (preferably, fuel is not injected into the engine during exhaust brake operation so that fuel is not passed through the engine unburned). The net result is that during exhaust brake operation the engine is absorbing power and not generating power. The engine exhaust brake is therefore an efficient braking system that can be used as a supplement to or a substitute for conventional wheel brakes, and may be used for repeated and extended braking operations.
Exhaust brakes may use special components, or may be realized using existing valvetrain components. Generally, exhaust braking requires components that can actuate (open) an exhaust valve independent of the normal valvetrain operation, under control of an exhaust brake system. Related art exhaust brake systems have included separate independent camshafts, rocker arms, or actuators to perform actuation of exhaust valves for exhaust braking. Related art devices have in the past actuated multiple exhaust valves in unison. This is of course the simplest operation conceptually, but simultaneous opening of both exhaust valves of a cylinder during exhaust braking has drawbacks.
A first drawback of the related art is the limitation imposed on an exhaust braking system due to excessive loads on valvetrain components. Because related art exhaust brakes typically rely upon a camshaft or camshaft pushrod to pivot an associated rocker arm as part of the exhaust brake valve operation, the pushrod must do the work of actuating the exhaust valves. The maximum exhaust braking performance is therefore limited by the load-handling ability of the pushrod or valvetrain components. This load is imposed upon the valvetrain by the pressure in each cylinder. The rocker arm (and any associated exhaust brake actuator) is acted upon by the pushrod in the related art and must open both exhaust valves at the same time, while being counter-acted by a high cylinder pressure. If a valve bridge is positioned across the exhaust valves and used to actuate the exhaust valves, the force required to open multiple valves is higher than the force required to open a single valve, imposing an even greater load upon the rocker arm and the pushrod. Exhaust braking performance has therefore in the past been limited to minimize problems such as, for example, wear, deformation, or breakage of pushrods, rocker arms, exhaust valve bridges, etc. If the pushrods cannot take the load, the exhaust valves may need to be opened earlier from top dead center (TDC) of a piston, thereby preventing exhaust braking from being as effective as it could optimally be.
Another drawback of related art combination exhaust brake systems which incorporate an exhaust restricter is the efficiency in which a cylinder may be charged with air for compression in exhaust braking. In combination exhaust brake operation, if the air is already somewhat compressed at the start of the compression stroke, more work must be done by the piston during the compression stroke. Pre-charging of a cylinder has already been done in a related art exhaust brake, U.S. Pat. No. 5,146,890 to Gobert et al. Because the air in the exhaust manifold is charged to a high pressure during the exhaust stroke of a cylinder, by opening an exhaust valve during the intake/compression stroke precharges the corresponding cylinder with high pressure air before or even during the compression stroke. Gobert discloses the opening of an exhaust valve during the latter part of an inlet stroke and during the first portion of a compression stroke. The high pressure air present in the exhaust manifold will flow into the cylinder, increasing the pressure in the cylinder. The pressure in the exhaust manifold exceeds the pressure of the intake air in the cylinder until about halfway through the compression stroke. However, Gobert discloses only a short duration opening of the exhaust valve during the compression stroke. FIG. 1 is a graph of the Gobert precharge system showing an exhaust manifold pressure 100, a cylinder pressure 102, a timing curve of the normal exhaust valve opening 105, the timing curve of the exhaust braking exhaust valve opening 108, and a timing curve of the precharge exhaust valve opening 112. As can be seen from the graph, the precharge exhaust valve opening 112 in Gobert occurs from about 170 degrees of crankshaft rotation to about 250 degrees of crankshaft rotation. It can be observed from the graph that the exhaust manifold pressure 100 exceeds the cylinder pressure 102 until about 275 to 290 degrees of crankshaft rotation, as shown by area 115 of the graph.
There remains a need in the art for improvements in engine braking systems.